On Fri, 30 Oct 2009 17:13:13 -0500, cavelamb
wrote:

Joe wrote:
I'm hoping someone might have a refference site for free wheeling prop
torque produced by a prop around the size of a 20X20 sailing at hull
speed. (9kts)

One goal on the boat we are designing and building will be an
electric drive system and we are leaning towards a power phase 150
drive.

http://www.uqm.com/propulsion_specs.php

It's a 200 HP perm magnet motor, so as you sail and the prop turns it
will generate power for the battery banks. These motors are now being
used in electric hummers in the Army. They need 420 volt DC using
two banks of batterys (35 batteries each bank) and will require 75 LBS
or torque to generate electricity.

Now I'm pretty sure the torque on a prop around the 22" size will be
way over 75 lbs but do not know exactly. Im hoping it's twice that at
least for gear reduction.

So I'm hoping someone might have, or know where I can get data on prop
torque. If not.... I guess I'm going to have to get a monkey wrench
and a scale and find a boat of the proper size to test.

Any suggestions?

Joe


As a starting point, it will probably make about the same amount of
torque as it uses under power - for the same prop rpm.


Probably a good starting place.

I've seen at least one 40 ft. sailboat with an auto alternator belted
to the prop shaft, the theory was that one could generator electricity
while sailing.

Another point is that there isn't a free lunch and I suspect that the
force necessary to turn a propeller generating X amount of power has
to come from somewhere. Probably in drag on the boat which will have
to be overcome by larger sails.

By the way, the statement "will require 75 LBS
or torque to generate electricity" is quite simply wrong as RPM of the
generator also enters into the equation. First because torque is not
measured in pounds, it is measured in force at some specified arm
length - Foot Pounds, Inch Ounces, etc. Secondly because movement of
the electrical generating fields is required. The specification should
have been some amount of horsepower, which is really another way to
specify speed and torque.



Cheers,

Bruce
(bruceinbangkokatgmaildotcom)